When certain halogenated pyrimidines such as bromodeoxyuridine (BrdUrd) and iododeoxyuridine (IdUrd) are incorporated into cellular DNA, the cells become more sensitive to ionizing radiation. This observation has led to several clinical studies over the years and recently at the NCI to evaluate whether selective sensitization of tumors could be achieved by IdUrd infusion followed by radiation. An important question arises in these studies regarding whether or not the drug actually is incorporated into cells. This study proposes to obtain information regarding this question by using: a) a IdUrd monoclonal antibody and HPLC assays to actually quantitate the amount of IdUrd in tumor compared to normal tissue; b) IdUrd monoclonal antibody to sort out S phase cells by flow cytometry and determine labelling index of human tumors; and c) evaluation of the possible potentiation of chemotherapy drugs by IdUrd incorporation. Further studies have questioned the role of low dose rate irradiation with halogenated pyrimidines.